1. Field of the Invention
This invention relates generally to a fuel cell system that heats an anode re-circulation gas for preventing water droplets from forming to improve system stability and, more particularly, to a fuel cell system that uses a heated cooling fluid directly from the stack to heat an anode re-circulation gas for preventing water droplets from forming to improve system stability.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer-electrolyte proton-conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. An automotive fuel cell stack may include about four hundred fuel cells. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack.
The fuel cell stack includes a series of flow field or bipolar plates positioned between the several MEAs in the stack. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode gas to flow to the anode side of the MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode gas to flow to the cathode side of the MEA. The bipolar plates also include flow channels through which a cooling fluid flows.
As is well understood in the art, the membranes within a fuel cell need to have a certain relative humidity so that the ionic resistance across the membrane is low enough to effectively conduct protons. During operation of the fuel cell stack, water by-product and external humidification may enter the anode and cathode flow channels. At low power demands, typically below 0.2 A/cm2, the water may accumulate within the flow channels because the flow rate of the reactant gas is too low to force the water out of the channels. As the accumulation of water increases, the flow channel may close off, and the reactant gas is diverted to other flow channels because the channels are in parallel between common inlet and outlet manifolds. Because the reactant gas may not flow through a channel that is blocked with water, the reactant gas cannot force the water out of the channel. Those areas of the membrane that do not receive reactant gas as a result of the channel being blocked will not generate electricity, thus resulting in a non-homogenous current distribution and reducing the overall efficiency of the fuel cell. As more and more flow channels are blocked by water, the electricity produced by the fuel cell decreases, where a cell voltage potential less than 200 mV is considered a cell failure. Because the fuel cells are electrically coupled in series, if one of the fuel cells stops performing, the entire fuel cell stack may stop performing.
In one known fuel cell design, anode exhaust gas is re-circulated back to the anode input so that un-reacted hydrogen in the exhaust gas can be reused. The anode exhaust gas from the stack is at an elevated temperature and is humidified as a result of the stack water by-product. Therefore, when the heated and humidified re-circulation anode exhaust gas is mixed with the fresh, dry and cool hydrogen at the anode input, the temperature differential causes the water vapor within the re-circulated anode exhaust gas to condense into liquid water droplets. The liquid water droplets then enter the anode reactant gas flow channels possibly causing cell stability problems as discussed above.